1. Field of the Invention
This invention relates to a circuit for generating a current signal having a given current waveform, in particular a trapezoidal current waveform with substantially evenly matched rise and fall times; and to a pulse transmitter circuit such as those in a data terminal transceiver.
In computer communication systems having a local data terminal which communicates with a local area network (LAN) or a larger global computer network, a transceiver is used between the data terminal and the network. This invention is particularly useful with those networks in which a pulse is transmitted by applying predetermined values of current pulses to the network cable, rather than transmitting voltage pulses of a given voltage level into the LAN cable.
In integrated circuit technology, a typical transmitter circuit receives input data in the form of a low voltage differential input signal from the data terminal, and must generate output data in the form of a higher power output signal having a particular waveform to the network. During the transmission of the particular waveform over media such as coaxial cable, noise and interference may distort the signal, causing errors in data transmission. The magnitude of these errors is affected by the shape of the waveform and by the transmitter output impedance relative to the network impedance.
To enable accurate discrimination between signal and noise pulses, it may be especially desirable that data pulses be trapezoidal rather than square, and have closely matched rise and fall times. The matching of these times is more critical than their actual value.
2. Description of the Prior Art
A well-known transmitter circuit transmits data in the form of a voltage signal having a trapezoidal voltage waveform. The transmitter circuit has a differential amplifier, a pair of current sources, an input switching device, and an on-chip integrating capacitor. The capacitor is connected between the negative input of the differential amplifier and the differential output. The first current source is connected between the negative input of the differential amplifier to ground. The second current source, having the opposite polarity and twice the magnitude, is connected via the switching device to the negative input of the differential input, in parallel with the first current source.
In response to high to low differential transitions applied to the switching device of the transmitter circuit, the switch opens, and the capacitor charges at a constant rate of I/C volts/sec. Likewise, in response to low to high transitions applied to the switching device, the switch closes, and two current sources combine to discharge the capacitor at a constant rate of -I/C volts/sec. Thus the known transmitter circuit generates a voltage signal having a trapezoidal waveform with matched rise and fall times.
Such a transmitter circuit is available from National Semiconductor as the DS 3662 Quad High Speed Transceiver.
Another circuit for providing a synthesized waveform which is a replica of an input pulse is taught by U.S. Pat. No. 3,898,571. The output pulse is obtained by summing the output voltages of a plurality of grated amplifiers. This circuit is of particular value in receivers which introduce pulse distortion via a logarithmic amplifier.
A disadvantage of the transmitter circuit described above is that there are no efficient conventional techniques to convert a voltage signal having a trapezoidal voltage waveform to a current signal having a similarly shaped current waveform without distorting the waveform characteristic. Such a voltage-to-current converter which produces a precise current waveform characteristic is particularly desirable for use in transmitter circuits, especially those used in transceivers which transmit data over a coaxial line by sinking current and receive data by sensing changes in voltage. More precisely transmitted current waveforms enable increased speed and accuracy of data communication between each terminal in the network.